The present invention relates to lifting equipment and, more particularly, to hydraulic jacks requiring a combination of high-speed operation over a large lifting range and high stability within a limited lifting range.
Hydraulic jacks are conventionally employed for raising loads under the influence of hydraulic pressure introduced into a hydraulic cylinder. The hydraulic pressure acts upon a piston slideably fitted within the hydraulic cylinder to force the piston outward and to thereby apply a force to an external object.
The distance through which a hydraulic jack is capable of moving its piston is limited primarily by the length of its cylinder. Hydraulic cylinders having a piston travel exceeding 100 feet are in routine use. The external force which a hydraulic jack can exert is equal to the area of its piston times the hydraulic pressure to which the piston is exposed.
As the distance through which a hydraulic piston must be moved becomes larger, it becomes more difficult to achieve high rigidity in supporting a load. In prior U.S. patent application Ser. No. 491,509, the disclosure of which is herein incorporated by reference and of which I am a co-inventor, a method is disclosed for unstacking shrunk-fit turbine wheels from a turbine shaft. In brief, the method employs an electric oven which can be installed, on site, about a turbine wheel to be removed while the shaft thereof is in a horizontal position. The oven is capable of injecting heat into the turbine wheel at a rate fast enough to expand the hub of the turbine wheel to the point that a clearance is developed in the shrunk-fit interface between the hub and the shaft before the shaft can expand sufficiently to eliminate the clearance. The referenced application also discloses a handling trolley which rapidly and accurately moves the released turbine wheel axially horizontally along the shaft until the hub is clear of the shrink-fit area. A similar but reverse procedure is employed in restacking the turbine wheels on the shaft.
In the disclosed referenced method, the turbine shaft is supported on fixed stands with its axis horizontally disposed. During the stacking and unstacking process, the fixed stands must be moved to different positions along the rotor. To permit moving the fixed stands, a jack is used to raise and support one end of the turbine rotor while a fixed stand is removed and repositioned. A substantial jack capacity is required to accurately handle a turbine rotor which may weigh, for example, in excess of 200 tons. At certain times it is desirable to combine the large jack capacity with high positioning precision over a small range of movement and, at other times, high speed with low precision over a large range of movement is desired. In addition, it is highly desirable that the jack be capable of stable support of the shaft for extended periods of time with enough rigidity that servicing operations such as, for example, machining of the shaft or attached elements, may be performed with the shaft supported by a bearing borne by the jack.